The present invention relates to a pneumatic tire in which the effect of residual cornering force due to the tread pattern is sufficiently suppressed to provide improved maneuverability.
To assure effective water drainage, the surfaces of ordinary roads have cants or are inclined in such a way that they are the highest in the central area while decreasing in height toward either shoulder of the road. Hence, a vehicle running straight on a straight road is subjected to a force exerted by the road which urges the vehicle to go down the cant, whereby it has a tendency to swerve in that direction.
As a matter of fact, depending on various tire factors such as construction and shape, the tire itself can be an effective source of lateral force, which is commonly referred to as "residual cornering force". If the residual cornering force balances with the lateral force developed by the cant, the vehicle will not swerve.
The residual cornering force is generally held to be highly susceptible to the direction of the belt placed under the tread, particularly with respect to the direction of the belt cords that lie radially outward of the tire. Accordingly, the direction in which the belt cords are attached is in turn determined by the cant of the road.
Consider, for example, the case where vehicles are required to run on the left side of the road; since the road surface on the left side is provided with a cant sloping downward to the left, some swerving of vehicles can be prevented if the cord in the outermost layer of the belt is attached in a direction sloping upward to the right as seen from the outside of the tire (see FIG. 9). In the other case where vehicles are required to run on the right side of the road, the road surface on the right side is provided with a cant sloping downward to the right; therefore, some swerving of vehicles can be prevented if the cord in the outermost layer of the belt is attached in a direction sloping upward to the left as seen from the outside of the tire (see FIG. 10).
In fact, however, the residual cornering force is occasionally developed by the tread pattern and it has been difficult to develop an appropriate amount of residual cornering force by simply adjusting the direction of attachment of belt cords.
A brief description of the developing mechanism of the residual cornering force is given here. Since the crown portion of the tire tread has a greater radius of rotation than the shoulder portion, a tractive force works on the blocks provided in the crown portion and a braking force on the blocks in the shoulder portion. Here, the blocks are twisted in the flow direction of the tread pattern, namely, in the direction determined by the inclination of the block sides relative to the direction of tread's width and the resulting torque will develop the residual cornering force.
Consider, for example, a prior art tire having the block pattern shown in FIG. 7(A). A block Bcr provided in the crown portion is inclined rearward in the direction of tire rotation toward the external part of the tread in the direction of its width, whereas a block Bsh in the shoulder portion is inclined forward in the direction of tire rotation toward the external part of the tread in the direction of its width, whereby a tractive force Ft works at the internal front end of block Bcr and a braking force Fb at the external front end of block Bsh. As a result, the two blocks Bcr and Bsh are twisted in outer directions and a torque T.sub.R rotating clockwise will develop in the area of the tread that is right to the center of the tread shown (the torque is counterclockwise in the tread area on the left side), whereby the self-aligning torque SAT will increase and, hence, the residual cornering force will increase as a whole (see FIG. 8(A)).
Consider also a prior art tire having the block pattern shown in FIG. 7(B). A block Bcr provided in the crown portion is inclined forward in the direction of tire rotation toward the external part of the tread in the direction of its width, whereas a block Bsh in the shoulder portion is inclined rearward in the direction of tire rotation toward the external part of the tread in the direction of its width, whereby a tractive force Ft works at the external front end of block Bcr and a braking force Fb at the internal front end of block Bsh. As a result, the two blocks Bcr and Bsh are twisted in inner directions and a torque T.sub.L rotating counterclockwise will develop in the area of the tread that is right to the center of the tread shown (the torque is clockwise in the tread area on the left side), whereby the self-aligning torque SAT will decrease and, hence, the residual cornering force will decrease as a whole (see FIG. 8(B)).
As described above, the residual cornering force taken as a whole varies with the flow direction of the tread pattern and, depending on the tread pattern, a great force will develop in an opposite direction to the residual cornering force determined by the direction of attachment of belt cords, causing the vehicle to slip down the cant of the road.
An object, therefore, of the present invention is to provide a pneumatic tire in which the effect to be caused by the tread pattern on the residual cornering force is sufficiently suppressed to insure enhanced maneuverability.